Unraveling\nReactive\nNitrogen Emissions in Heavy-Duty\nDiesel Vehicles across Evolving Standards and Cheating Tactics

Reactive nitrogen (Nr) emissions\nsignificantly affect\nair quality\nand the nitrogen cycle in ecosystems. Heavy-duty diesel vehicles (HDDVs),\nas major sources of these emissions, exhibit complex emission characteristics\nbecause of the combined effects of different driving conditions and\naftertreatment technologies. This study first investigated the emission\nfactors (EFs) of Nr species, including NO, NO<sub>2</sub>, HONO, N<sub>2</sub>O, and NH<sub>3</sub>, from HDDVs under different emission\nstandards (China IV/V/VI) and cheating strategies, with a particular\nfocus on the impact of selective catalytic reduction (SCR) systems.\nVehicles employing water injection cheating present NO, NO<sub>2</sub>, and HONO EFs that are consistent with the China III standards,\nsignificantly undermining the effectiveness of Nr emission control.\nThe evolution of SCR technology in China IV, V, and VI standards has\ngenerally led to substantial reductions in NO, NO<sub>2</sub>, and\nHONO emissions, yet the integration of ammonia slip catalysts (ASC)\nsystems in China VI vehicles presents new challenges. While ASCs have\nsuccessfully reduced NH<sub>3</sub> slip to an average of 17 ±\n12 mg/km, they have also caused a 6–13-fold increase in N<sub>2</sub>O emissions compared with those of China IV and V vehicles,\nreaching levels of 205 ± 85 mg/km. Additionally, China VI vehicles\nexhibit a marked increase in the HONO/NOx ratio, which increases from\n0.9% in China V to 4.6%. These increases are attributed to high-temperature\noxidation of NH<sub>3</sub> within the ASC catalyst, leading to undesirable\nbyproducts. The temporal dynamics of Nr emissions under real-world\ndriving conditions further reveal that the effectiveness of aftertreatment\ntechnologies and their selectivity toward byproducts vary depending\non the driving mode. This variability underscores the need for further\noptimization of the SCR and ASC technologies to balance the control\nof all the reactive nitrogen species effectively.